1. Field of the Invention
This invention relates to the field of electrophotographic, and more particularly, to an electrophotographic apparatus wherein a monochromatic light beam is used to print a document from an electrical data defined image.
2. Prior Art
It is well known in the prior art to use a monochromatic light beam to scan a photosensitive material for facsimile recording and generation of recorded data. Prior devices have utilized a laser light source to generate the monochromatic light beam. The light beam is then intercepted by an acousto-optic light deflector which deflects the beam into zero order, first order and higher order beams in accordance with a phenomenon called RomanNath scattering and Brillouin scattering. One of the optical higher order beams, (usually the first order beam) designated as the working beam, is used for facsimile recording on a photosensitive material. For control purposes, the acousto-optic light deflector is driven by a facsimile signal modulator. When the above device and/or method is used in an electrophotographic device, the photosensitive material is toned, developed and the toned image is then transferred to a receiving media.
Although the prior art devices function satisfactorily for the intended purpose, the systems have several problems and drawbacks which render them unacceptable when used in an electrophotographic device. Probably the most pressing drawback with the prior art systems is the inability to maintain the power level of the working beam (i.e., the beam which intercepts the photosensitive material) constant. As is well known to those skilled in the art, there is a strong relationship between the power contents of the working beam and the quality of a copy (i.e., the output) which is derived from a latent image on the photosensitive material. Generally, if the power contents of the working beam remains constant, the photosensitive material will be evenly discharged and the quality of a subsequent copy (i.e., the output) will be acceptable. Conversely, if the power contents of the working beam is not constant then the photosensitive material will be unevenly discharged and the quality of the subsequent copy (i.e., the output) will be unacceptable.
Several methods and devices have been practiced in the prior art to maintain a constant power level in the working beam. One of these methods is to insure that the power supplies which are driving the laser and the facsimile generator are maintained at a constant level. However, due to degradation (over a period of time) in the components of the laser and the facsimile generator, even if the power from the power supplies is constant, the output from the laser and facsimile generator varies. So, in order for the power supply output to maintain power at a constant level, (i.e., without drifting) expensive components and expensive circuits are required for its design.
Another approach which has been used by the prior art to solve the aforementioned problem is to stabilize the laser. Several exotic and expensive methods, such as baking, etc., have been tried without avail. The lack of success stems from the fact that the laser and its components deteriorate with time while the method used to stabilize the laser is static.
Stated another way, the laser beam varies dynamically due to components deterioration etc. However, the methods which are used to stabilize the laser beam are non dynamic.
Still another approach which is practiced in the prior art to solve the aforementioned problem, is to derive a control signal from a higher order non working beam and use said derived signal to control the facsimile generator. Although in some cases this method was a significant improvement over the prior art, it is affected by problems.
The first problem is that although the beam whose power level is to be maintained constant is the working beam, the beam whose power is sensed and is used to control the facsimile generator so as to modify the power level or power contents of said working beam is a beam other than the working beam. With this method of control, it is extremely difficult, if not impossible, to be certain that the power level or power contents of the working beam is constant.
Another problem with the prior art systems is that the power level or power contents of beams beyond the first order beam (i.e., higher order beams) is extremely low for sensing and, therefore, requires extremely sensitive and expensive electronic circuits to control the facsimile generator.
Still another problem is that the power content of the working beam is adjusted continuously. As is well known to those skilled in the art, in an electrophotographic machine the laser beam is used to selectively discharge incremental areas on the photosensitive material so as to print a latent image of an original. During the discharge of these incremental areas, it is important that the power content of the working beam remain constant. However, with the prior scheme wherein the working beam is continuously adjusted, the power level of the beam is always in a state of flux (i.e., continuous change) during incremental discharge of the photosensitive material. With this defect (i.e., the continuous adjustment of power level of the working beam), it is possible that the quality of the copies derived from a device embodying the described scheme is less than acceptable.